1. Field of the Invention
The present invention relates to apparatus and methods for inclusion in, illustratively, a Coriolis mass flow rate meter that substantially eliminate temperature induced measurement errors which might otherwise be produced by performance differences existing between two separate input channel circuits contained in the meter.
2. Description of the Prior Art
Currently, Coriolis meters are finding increasing use in a wide variety of commercial applications as an accurate way to measure the mass flow rate of various process fluids.
Generally speaking, a Coriolis mass flow rate meter, such as that described in U.S. Pat. No. 4,491,025 (issued to J. E. Smith et al on Jan. 1, 1985 and owned by the present assignee hereof--hereinafter referred to as the '025 Smith patent), contains one or two parallel conduits, each typically being a U-shaped flow conduit or tube. As stated in the '025 Smith patent, each flow conduit is driven to oscillate about an axis to create a rotational frame of reference. For a U-shaped flow conduit, this axis can be termed the bending axis. As process fluid flows through each oscillating flow conduit, movement of the fluid produces reactionary Coriolis forces that are orthogonal to both the velocity of the fluid and the angular velocity of the conduit. These reactionary Coriolis forces, though quite small when compared to a force at which the conduits are driven, nevertheless cause each conduit to twist about a torsional axis that, for a U-shaped flow conduit, is normal to its bending axis. The amount of twist imparted to each conduit is related to the mass flow rate of the process fluid flowing therethrough. This twist is frequently measured using velocity signals obtained from magnetic velocity sensors that are mounted to one or both of the flow conduits in order to provide a complete velocity profile of the movement of each flow conduit with respect to either the other conduit or a fixed reference. In dual conduit Coriolis meters, both flow conduits are oppositely driven such that each conduit oscillates (vibrates) as a separate tine of a tuning fork. This "tuning fork" operation advantageously cancels substantially all undesirable vibrations that might otherwise mask the Coriolis force.
In such a Coriolis meter, the mass flow rate of a fluid that moves through the meter is generally proportional to the time interval (the so-called ".DELTA.t" value) that elapses between the instant one point situated on a side leg of a flow conduit crosses a pre-determined location, e.g. a respective mid-plane of oscillation, until the instant a corresponding point situated on the opposite side leg of the same flow conduit, crosses its corresponding location, e.g. its respective mid-plane of oscillation. For parallel dual conduit Coriolis mass flow rate meters, this interval is generally equal to the phase difference between the velocity signals generated for both flow conduits at the fundamental (resonant) frequency at which these conduits are driven. In addition, the resonant frequency at which each flow conduit oscillates depends upon the total mass of that conduit, i.e. the mass of the conduit itself, when empty, plus the mass of any fluid flowing therethrough. Inasmuch as the total mass varies as the density of the fluid flowing through the conduit varies, the resonant frequency likewise varies with any changes in fluid density and, as such, can be used to track changes in fluid density.
For some time, the art has taught that both velocity signals are processed through at least some analog circuitry in an effort to generate output signals that are proportional to the mass flow rate of the process fluid. In particular, the output signal associated with each velocity sensor is ordinarily applied through analog circuitry, e.g. an integrator followed by a zero crossing detector (comparator), contained within a separate corresponding input channel. In this regard, see illustratively U.S. Pat. No. 4,879,911 (issued to M. J. Zolock on Nov. 14, 1989), U.S. Pat. No. 4,872,351 (issued to J. R. Ruesch on Oct. 10, 1989), U.S. Pat. No. 4,843,890 (issued to A. L. Samson et al on Jul. 4, 1989) and U.S. Pat. No. 4,422,338 (issued to J. E. Smith on Dec. 27, 1983)--all of which are also owned by the present assignee hereof. While the various approaches taught in these patents provide sufficiently accurate results in a wide array of applications, the meters disclosed in these references, as well as similar Coriolis meters known in the art, nevertheless suffer from a common drawback which complicates their use.
Specifically, Coriolis mass flow meters operate by detecting what amounts to be a very small inter-channel phase difference between the signals produced by both velocity sensors, i.e. the .DELTA.t value, and transforming this difference into a signal proportional to mass flow rate. While, at its face, a .DELTA.t value is obtained through a time difference measurement, this value, in actuality, is also a phase measurement. Using such a time difference measurement conveniently provides a way to accurately measure a manifestation of a phase difference appearing between the velocity sensor signals. In Coriolis meters currently manufactured by the present assignee, this difference tends to amount to approximately 130 .mu.sec at maximum flow. Each input channel in a Coriolis meter imparts some internal phase delay to its input signal. While the amount of this delay is generally quite small, it is often significant when compared to the small inter-channel phase difference, i.e. 130 .mu.sec or less, that is being detected. Currently available Coriolis meters have relied on assuming that each input channel imparts a finite and fixed amount of phase delay to its corresponding velocity signal. As such, these Coriolis meters generally rely on first measuring, at a true zero flow condition occurring during meter calibration, either the inter-channel phase difference (.DELTA.t) or the indicated mass flow rate. Subsequently, while metering actual flow, these meters will then subtract the resulting value, in some fashion, from either the measured .DELTA.t or mass flow rate value, as appropriate, in order to generate an ostensibly accurate mass flow rate value for the process fluid then flowing therethrough.
Unfortunately, in practice, this assumption has proven to be inaccurate. First, not only does each input channel often produce a different amount of internal phase delay with respect to the other, but also the phase delay that is produced by each channel is temperature dependent and varies differently from one channel to the other with corresponding changes in temperature. This temperature variability results in a temperature induced inter-channel phase difference. Because the measured phase difference (.DELTA.t) that results from actual flow through the meter is relatively small, then an error in the measured phase difference between the velocity signals and attributable to the temperature induced inter-channel phase difference can, in certain instances, be significant. This error is generally not taken into account in currently available Coriolis mass flow rate meters. In certain situations, this error can impart a noticeable temperature dependent error into mass flow rate measurements, thereby corrupting the measurements somewhat.
In an effort to avoid this error, one well known solution in the art is to shroud an installed piped Coriolis meter, including its electronics, with a temperature controlled enclosure. This approach, which prevents the meter from being exposed to external temperature variations and maintains the meter at a relatively constant temperature while it is in operation, greatly increases the installed cost of the meter and is thus not suited for every application. Hence, in those applications where installed cost is a concern, this approach is generally not taken. Specifically, in those applications and particularly where the meter is to be sited indoors and not exposed to wide temperature variations, then the measurement error which results from the temperature induced inter-channel phase difference, while generally expected, tends to remain quite small and relatively constant. As such, this error is usually tolerated by a user. Unfortunately, in other applications where the meter is not housed in a temperature controlled enclosure, such as outdoor installations where the meter is expected to experience wide fluctuations in operating temperature, the error generally varies and can become significant, and thus needs to be taken into account.
Apart from errors arising from temperature induced inter-channel phase differences, many currently available Coriolis mass flow rate meters also disadvantageously exhibit an additional source of measurement inaccuracy related to temperature. In particular, Coriolis meters generally measure the temperature of the flow conduit and, owing to changes in flow conduit elasticity with temperature, accordingly modify a meter factor value based upon the current temperature of the conduit. This meter factor, as modified, is then subsequently used to proportionally relate the inter-channel phase difference (.DELTA.t) value to mass flow rate. Flow conduit temperature is measured by digitizing an output of a suitable analog temperature sensor, such as a platinum RTD (resistive temperature device), that is mounted to an external surface of a flow conduit. The digitized output usually takes the form of a frequency signal, oftentimes produced by a voltage-to-frequency (V/F) converter, that is totalized (counted) over a given timing interval to yield an accumulated digital value that is proportional to flow conduit temperature. Unfortunately, in practice, V/F converters usually exhibit some temperature drift which, based upon the magnitude of a change in ambient temperature, could lead to an error, amounting to as much as several degrees, in the measurement of flow conduit temperature. This error will, in turn, corrupt the mass flow rate.
A solution proposed in the art to ostensibly deal with temperature dependent variations in the performance of the input channels of Coriolis meters is taught in U.S. Pat. No. 4,817,448 (issued to J. W. Hargarten et al on Apr. 4, 1989 and also owned by the present assignee hereof--hereinafter referred to as the '448 Hargarten et al patent). This patent discloses a two channel switching input circuit for use in a Coriolis meter. In particular, this circuit includes a two-pole two-throw FET (field effect transistor) switch located between the outputs of the velocity sensors and the inputs to both of the channels. In one position, the FET switch connects the outputs of the left and right velocity sensors to corresponding inputs of the left and right channels, respectively; while in the opposite position, these connections are reversed. The switch is operated to change its position at every successive cycle of flow conduit movement. In this manner, the output of each velocity sensor is alternately applied to both channels in succession. Over a two cycle interval, appropriate time intervals are measured with respect to the velocity waveforms applied to both channels and then averaged together to yield a single time interval value from which errors attributable to each individual channel have been canceled. This resulting time interval value is then used in determining mass flow rate through the meter.
While this solution does indeed substantially eliminate temperature induced inter-channel phase differences, it possesses a drawback which limits its utility somewhat. Specifically, this input circuits in the apparatus taught in '448 Hargarten et al patent do not include integrators. Owing to the lack of any low pass filtering that would have been provided by integrators, these input circuits are therefore susceptible to noise. Unfortunately, the switching scheme taught in this patent does not permit integrators to be included in the switched portion of the input circuitry, hence requiring that, to provide noise immunity, an integrator must be located after the FET switch. Unfortunately, in this location, the phase delay inherent in the integrator can not be readily compensated, if at all. Inasmuch as the integrator disadvantageously tends to provide the largest source of phase delay in the input circuitry, inclusion of such an integrator would add an error component, i.e. an uncompensated phase delay, to the measured .DELTA.t values. Moreover, this phase delay would also vary with temperature changes. Consequently, the resulting measured flow rate values would contain an error component. Thus, it became apparent that the solution posed in the '448 Hargarten et al patent has limited applicability to relatively noise-free environments.
Therefore, a need exists in the art for a Coriolis meter that provides accurate flow and flow rate output values that are substantially insensitive to ambient temperature variations and hence does not appreciably exhibit adverse temperature affects an could provide appreciable noise immunity. Such a meter should possess negligible, if any, temperature induced measurement inaccuracies over relatively wide variations in ambient temperature thereby permitting the meter to be used to provide highly accurate flow measurements in a wide variety of applications and particularly without a need to house the meter in a temperature controlled enclosure. Advantageously, the increased measurement accuracy provided by such a meter and the attendant installed cost savings associated therewith would likely broaden the range of applications over which such a meter could be used.